The invention relates to a display device comprising a liquid crystal display with at least one polarizer, which liquid crystal display comprises two parallel supporting plates having facing surfaces spaced a distance d from each other and on one surface of which a pattern of line electrodes is provided and on the other surface of which a pattern of column electrodes is provided, the line electrodes crossing the column electrodes and the crossings forming a matrix of display cells, a layer of a nematic liquid crystal with a cholesteric addition being present between the supporting plates, the liquid crystal having a positive dielectric anisotropy and a natural pitch p.
Such display devices are known and are conventionally operated in multiplex drive with electric voltages according to the so-called r.m.s. mode. The way of driving is described by Alt and Pleshko in IEEE Trans. El. Dev., Vol. ED 21, 1974, pp. 146-155 and is considered the most widely used manner of driving a liquid crystal display devices. The maximum number of lines n which can be driven with an acceptable contrast ratio by means of this method is determined by the relation: ##EQU1## wherein V.sub.2 is the required r.m.s. voltage across a display cell to switch it to the "on" condition, and V.sub.1 is the r.m.s. voltage at which the display cell is in the "off" condition. Accordingly as V.sub.2 and V.sub.1 are brought closer together, a larger number of lines n can be driven. Of course this requires a steep threshold in the transmission/voltage characteristic of the display cell. However, as n increases, the time in which the desired contrast ratio is achieved also increases.
A steep threshold, however, does not say anything about the speed of the optical effect which brings a display cell from the "off" condition to the "on" condition (the so-called rise time). This also applies to the speed of the optical effect which brings a display cell from the "on" condition back to the "off" condition (the so-called decay time). As is known in the known liquid crystal display devices operated according to the r.m.s. mode the rise time and decay time of optical effects are considerably larger than the time necessary to write one complete frame (frame time). This leads to long switching times from the "off" condition to the "on" condition of a display cell, and conversely. Therefore such devices are not so suitable for displaying information with great information density rapidly varying in time, for example, as is the case in television and video. A further great disadvantage of the known display devices of the twisted nematic type is that the contrast of the displayed picture depends considerably on the angle at which the image is viewed. As the viewing angle increases the contrast decreases considerably. In this connection it is pointed out that the contrast in itself already is a problem in such display devices having more than 100 lines, i.e. display devices having a multiplexing ratio of more than 1:100.